TrFO and TFO are mechanisms to improve voice quality in wireless networks by avoiding double transcoding. TrFO uses out-of-band negotiation between terminations to select a common codec, while TFO uses in-band negotiation. If TrFO negotiation fails to find a common codec, TFO may be used as a fallback. Both aim to enable direct transmission between terminations when their codecs match, avoiding intermediate transcoding functions. The document describes the functions, processes, and benefits of each approach.

1. Feature Description Functional Features
HUAWEI UMTS Circuit-Switched Core Network Table of Contents
Table of Contents
Chapter 5 TrFO and TFO ................................ ................................ ................................ .......... 5-1
5.1 Overview of TrFO and TFO ................................ ................................ ........................... 5-1
5.2 TrFO Functions ................................ ................................ ................................ ................. 1
5.3 TFO Functions ................................ ................................ ................................ .................. 3
5.4 TrFO and TFO ................................ ................................ ................................ .................. 4
5.5 Function of MGW during TrFO/TFO ................................ ................................ .................. 4
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2. Feature Description Functional Features
HUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO
Chapter 5 TrFO and TFO
5.1 Overview of TrFO and TFO
In the radio network, in order to improved network utility efficiency and user capacity, it
is necessary to compress voice. Two terminations in the network supporting different
codec types cannot interconnect with each other until two times of transcoding are
implemented. During the transcoding process, the voice codec type needs to be
decoded and then re-encoded, so voice is damaged and voice quality is lowered.
Transcoder Free Operation (TrFO) and Tandem Free Operation (TFO) are two
negotiation mechanisms to ensure voice quality in wireless networks. TrFO is
applicable to UMTS and compliant with the specification 3GPP TS 25.953; while TFO is
applicable to GSM and UMTS and compliant with the specifications 3GPP TS 22.053,
3GPP TS 23.053 and 3GPP TS 28.062.
UMG8900 supports both TrFO and TFO to reduce transcoding time and improve voice
quality by cooperating with (G)MSC Server in the core network and equipments of the
access network.
In TrFO, the point-to point out-of-band negotiation about voice coding/decoding is
implemented based on Out of Band Transparent Control (OoBTC). Out-of-band
negotiation means that the channel used for the negotiation between two terminations
is independent from the service flow channel, and the point-to-point codec type is
selected based on the BICC protocol.
TFO adopts in-band negotiation. The in-band negotiation means the service flow
channel is used for negotiation. TC participates in the whole negotiation process.
5.2 TrFO Functions
The application of TrFO in the entire radio system network is shown in Figure 5-1.
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3. Feature Description Functional Features
HUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO
Oo Codec
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Figure 5-1 TrFO application network structure
In the TrFO mode, the calling ME performs out-of-band negotiation with the called ME.
If they support the same codec type, transcoding is not needed in the service bearer
set-up process, that is, transcoders are not necessary to add.
In the TrFO mode, voice codec type negotiation is implemented by (G)MSC Server
nodes. A typical negotiation process involving more than two (G)MSC Servers is
detailed as below.
1) The originating (G)MSC Server (O-MSC) sends the codec types and an option list
(IAM messages) supported, which are arranged as preference.
After receiving the information from the originating (G)MSC Server node, the
intermediate (G)MSC Server node deletes the options it does not support and then
continue to send the list. The preference parameter in the list is not changed.
The termination (G)MSC Server node (T-MSC) analyzes the list, then selects the codec
type with the highest preference (an APM message, containing the selected codec
type and optional codec types list), and finally returns its selection result. Till now,
the negotiation process is completed.
If no commonly supported code type can be selected during negotiation, transcoders
need to be inserted for transcoding.
The OoBTC negotiation can be classified into three types. And different negotiation
processes correspond to different services:
Basic negotiation process (originating A, intermediate B and remote C): In this
process, basic calling services are involved.
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4. Feature Description Functional Features
HUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO
Bearer modification process (originating A, remote B and remote C): C needs to
modify B selection and bearer. The services concerned are CFU, CFB, CFNRy,
CFNRc, CD, ECT and IN (with a more completed process).
Three-party negotiation process (originating A, remote B and remote C): CCD
(Conference Call Device) is located at A. Firstly set up the link between A and B (A,
B, Codec x), and then add C, which is only connected with A (A, C, Codec y).The
concerned services are CW, CH and MPTY.
5.3 TFO Functions
In GSM or UMTS, two times of transcoding must be implemented when an MS
accesses another MS or an MS accesses a UE as shown in Figure 5-2:
Tandem
Transcoding
LM
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§ Transcoding Transcoding §
MS UE MS UE
Function Function
Encoding e coding
Encoding e coding
compressed ITU-T G.711 compressed
§
speech -law U-law speech
Figure 5-2 Typical cascaded operations of TCs
When an MS accesses another MS, the voice transcoding process is described as
follows:
The originating MS encodes voice signals, and then sends them to the P MN A it
resides in through radio links. The local TC decodes the compressed voice and
according to ITU-T G.711, converts the decompressed voice to A-law or U-law PCM
voice that is then transferred to the radio network P MN B across the fixed network.
The TC of P MN B encodes the voice and the P MN B sends the compressed voice to
another MS.
In the above networking application, the two TCs are in cascading operation status. As
the voice is encoded and decoded twice in the call process, the voice signals shall be
damaged especially in a low-rate TC.
To minimize the damage on voice and improve voice quality, the TC functions are
disabled at the P MN A and P MN B so that the voice signals are transparently
transmitted between the originating and terminating MSs provided that the originating
and terminating MS uses the same TC. This is the function of the TFO protocol. In this
case, voice signals are only coded at the originating MS and decoded the terminating
MS once, thus improving the voice quality.
TFO affords the following functions:
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5. Feature Description Functional Features
HUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO
Establishing a channel for transparent transmission between two TCs.
Providing in-band signaling between two TCs for TFO operation control.
Transferring information of TC configurations such as ACS and SCS between two
TCs.
Making codec types matched.
Enabling and implementing TFO if the same codec type is selected by both sides.
Restoring the TC cascading operation status quickly and seamlessly from TFO
status if necessary.
To make codec types matched, the method of optimising TC can be used. The
optimising results may require TFO. In this case, other functional entities such as RNC
may be required to participate in enabling TFO.
TFO offers multiple benefits including:
Avoid repeated TC operations and improved voice quality. The effect is especially
obvious in the presence of low-rate transcoding.
Occupy less P MN resources. During TFO, 16kbit/s or 8kbit/s multiplexed voice
signals that are compressed are transferred across networks.
Save the power of network equipments. During TFO, TC is disabled.
Possibly reduce the point-to-point transmission delay.
At present, UMG8900 supports TFO of AMR transcoding only. For details about TFO
message sequence, please see 3GPP TS 28.062 specifications.
5.4 TrFO and TFO
The association and difference between TrFO and TFO are:
If the OoBTC fails to establish the TrFO and transcoders are required, then
in-band TFO may be used for negotiation.
TFO is activated either at set-up or during the communication phase.
In-band TFO shall be used for interworking with the 2G systems, and TrFO shall
used for pure 3G network application.
In-band TFO shall be the fallback mechanism when transcoders cannot be
avoided,
R99 set limitations of TrFO. The BICC protocol set1 only support AA 1 and AA 2, and
does not support bearer modification or repeated codec negotiation. For example, if a
node has no BC modification capability, you have to release the bearer link already set
up and then set up a new one.
5.5 Function of MGW during TrFO/TFO
TrFO is firstly originated by the user ME. And only when there is no commonly
supported code type during negotiation, is MGW used. (G)MSC Server initiates
transcoder insertion, and under the control of the (G)MSC Server, MGW implements
transcoder insertion according to the H.248 messages received.
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6. Feature Description Functional Features
HUAWEI UMTS Circuit-Switched Core Network Chapter 5 TrFO and TFO
During TFO, TC is responsible for the whole negotiation process, including making the
decision whether to perform transcoding.
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